1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is roller cone-type drill bit 20, shown in further detail in FIG. 2.
Roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22 and able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 may comprise tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
Significant expense is involved in the design and manufacture of drill bits to produce drill bits with increased drilling efficiency and longevity. For more simple bit designs, such as fixed cutter bits, models have been developed and used to design and analyze bit configurations having optimally placed cutting elements, a more balanced distribution of force on the bit, and a more balanced distribution of wear on the cone. These force-balanced bits have been shown to be long lasting and effective in drilling earth formations.
Roller cone bits are more complex in design than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones. Each of the roller cones independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the roller cones rotate independent of each other, the rotational speed of each cone is likely different. For a given cone, the cone rotation speed can be determined from the rotational speed of the bit and the effective radius of the xe2x80x9cdrive rowxe2x80x9d of the cone. The effective radius of a cone is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone. The cutting elements which extend axially the farthest from the axis of rotation of the cone are generally located on a so-called xe2x80x9cdrive rowxe2x80x9d. In some configurations, the cutting elements on the drive row are located to drill the fall diameter of the bit. In such cases, the drive row may be interchangeably referred to as the xe2x80x9cgage rowxe2x80x9d.
Adding to the complexity of roller cone bit designs, cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modem roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG. 3A and further detailed in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone drill bits is desired to permit high insert protrusion to achieve competitive rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby, further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have been largely developed through trial and error. For example, if its been shown that a prior art bit design leads to cutting elements on one cone of a bit being worn down faster that the cutting elements on another cone of the bit, a new bit design might be developed by simply adding more cutting elements to the faster worn cone in hopes of reducing wear on each of the cutting elements on that cone. This trial and error method of designing roller cone drill bits has led to roller cone bits with cutting elements unequally distributed between the cones, wherein the number of cutting elements on one cone of the bit differs by three or more from the number of cutting elements on another cone of the bit. In some cases, especially those involving cutting structures comprising intermeshing teeth, the difference between the number of cutting elements on each cone is significantly more than three. In some prior art bit designs, the unequal distribution of the number of cutting elements between the cones may result in an unequal distribution of force, strain, stress, and wear between the cones, which can lead to the premature failure of one of the cones. In other prior art bit designs, the unequal distribution of the number of cutting elements between the cones may result in an unequal distribution of contact with the formation between the cones or an unequal distribution of volume of formation cut between the cones.
One example of a prior art roller cone bit configuration considered effective in drilling well bores is shown in FIGS. 3A-3D. In FIG. 3A, the profiles of each of the cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. This drill bit comprises a bit body 100 and three roller cones 110 attached to the bit body 100 such that each roller cone 110 is able to rotate with respect to the bit body 100 about an axis oblique to the bit body 100. Disposed on each of the cones 110 is a plurality of cutting elements 112 for cutting into an earth formation. The cutting elements are arranged about the surface of each cone in generally circular, concentric rows arranged substantially perpendicular to the axis of rotation of the cone, as illustrated in FIG. 3C. In this example, the rows of cutting elements are arranged so that cutting elements on adjacent cones intermesh between the cones. In this example, the cutting elements 112 comprise milled steel teeth with hardface coating applied thereon.
As is typical for milled tooth roller cone bits with intermeshing teeth, the teeth in this example are arranged in three rows 114a, 114b, and 114c on the first cone 114, two rows 116a and 116b on the second cone 116, and two rows 118a and 118b on the third cone 118. The first row 114a on the first cone 114 is located at the apex of the cone and is typically referred to as the spearpoint. Referring to FIG. 3C, the first row 114a of the first cone comprises four teeth spaced about the apex of the cone as shown in the table at 120 and illustrated in the spacing diagram at 134. The second row 114b on the first cone 114 comprises nine teeth spaced apart as shown in the table at 122 and illustrated in the spacing diagram at 136. The third row 114c on the first cone 114 comprises nine teeth spaced apart as shown in the table at 124 and illustrated in spacing diagram at 138. The first row 116a on the second cone 116 comprises five teeth spaced apart as shown in the table at 126 and illustrated in the spacing diagram at 140. The second row 116b on the second cone 116 comprises nine teeth spaced apart as shown in the table at 128 and illustrated in the spacing diagram at 142. The first row 118a on the third cone 118 comprises seven teeth spaced apart as shown at 126 and illustrated in the spacing diagram at 144. The second row 118b on the third cone 118 comprises eleven teeth spaced apart as shown at 128 and illustrated in the spacing diagram at 146.
This prior art drill bit has a total of fifty-four teeth, wherein twenty-two teeth are disposed on the first cone, fourteen teeth are disposed on the second cone, and eighteen teeth are disposed on the third cone. The greatest difference in the number of teeth on any two cones for this prior art bit is eight. Thus the distribution of the teeth on this bit is significantly imbalanced, as is typical for prior art roller cone bit designs.
The invention comprises a roller cone drill for drilling earth formations. The drill bit comprises a bit body and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. The drill bit further comprises a plurality of teeth disposed on each of the roller cones such that the number of teeth on each cone differs by two or fewer from the number of teeth on each of the other cones. In one preferred embodiment, the drill bit comprises three roller cones. In another preferred embodiment, the teeth of the bit are arranged on each cone so that teeth on adjacent cones intermesh between the cones. In another preferred embodiment, the drill bit comprises a first cone, a second cone, and a third cone, and the number of teeth on each of the cones is 17, 16, and 18, respectively.